Thermally stabilized metal article and process of making it



United States Patent 3 198,610 THERMALLY STABILIZED METAL ARTICLE AND PROCESS OF MAKING lT Marshall G. Whitfield, 1 Paul St., P.O. Box 293, Bethel, Conn. No Drawing. Filed Oct. 11, 1961, Ser. No. 144,290 8 Claims. (Cl. 29-197) It has hitherto been understood that tungsten and molybdenum bodies and ferrous alloys rich in tungsten or molybdenum could be given a degree of thermal stabilization by being coated with aluminum. Whereas the oxides of tungsten and molybdenum sublime at high temperaturcs, the coated bodies are in some way protected from this action. The aluminum coating, in part at least, appears to diffuse into a coact with the molybdenum and tungsten to form a covering or protective layer. It has also been suggested that in coating the base bodies with the aluminum, a preliminary interface coating of nickel might be employed. The formation of the ultimate protective coating occurs upon the application of high heat, which may occur in service, but which is preferably carried on as a separate treatment.

However, it is also known that spalling or scaling-off of the ultimate protective layer can and does occur under severe conditions of heat cycling, and hitherto the art has had no cure for thi defect. Efforts to alleviate the condition by cutting down on the weight or thickness of the applied aluminum coating have only been partially successful, and have the additional disadvantage of minimizing the protective effect of the ultimate layer. Attempts to introduce aluminum into the surfaces of the articles by vapor deposition, or the use of porous surface layers, have not been found adequate, for example, in view of the requirements of the higher output jet engines or in rocketry. The difficulty with spalling is especially severe where the articles themselves are made of ferrous alloys rich in molybdenum or tungsten.

A primary object of the invention is to provide a firmly bonded alloy coating on metallic objects of the character hereinafter set forth, which will improve their resistance to oxidation, thermal shock and erosion at elevated temperatures, and without the disadvantage noted above.

It is an object of the invention to provide protected articles in which spalling has been minimized, but in the manufacture of which relatively heavy or thick layers of aluminum can be used.

Another object of the invention is to provide protective coatings for metal objects consisting of highly alloyed materials, particularly intended for turbine vanes or blades (in connection with which exemplary embodiment the invention will be described) but not limited thereto, and applicable to a wide range of other articles which must withstand high degrees of heat.

These and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, are accomplished by those methods and in those novel articles of which the aforesaid exemplary embodiment will now be described.

The teachings of this application pertain not only to bodies of molybdenum and tungsten and their alloys, but also to bodies of niobium or columbium and its alloys as well. The base articles of the invention may thus be stated to be articles made of metal'chosen from a class consisting of molybdenum, tungsten, niobium, and mixtures thereof, ferrous alloys containing a preponderant amount of one or a combination of the foregoing elements, and alloys containing a preponderant amount of the foregoing elements and in which the other alloying ingredients consist essentially of nickel, cobalt, or a combination there of. All of these materials must be protected from oxida- 3,198,610 Patented Aug. 3, 1965 tion and must be thermally stabilized in order to make use of their properties, particularly at temperatures in excess of about 1800 F.

It has been found that the objects of this invention may be attained by coating the base articles, preparatory to applying aluminum to their surfaces, with a ferrous alloy consisting essentially of a minor amount of nickel and a major amount of iron as hereinafter set forth. This intermediate layer of alloy may contain minor amounts of other ingredients such as chromium.

The outer coating as initially applied will be a coating of aluminum, preferably pure aluminum; but it does not amount to a departure from the spirit of this invention to employ aluminum containing minor amount of other alloying ingredients such as silicon, or magnesium.

When articles so treated are coated with aluminum and then subjected to heat, suprisingly, a very high degree of thermal stabilization is attained including resistance to oxidation and thermal fatigue, and the spalling or scalingoff of the protective coating is greatly minimized. The reasons why this is so are not clearly understood. Under conditions of high heat a diffused protective coating is still formed. First it would not be through a priori that thermal stabilization would be retained or improved by interposing between the aluminum and the base body an interface layer free from molybdenum, tungsten, columbium or niobium but rich in iron. Second, without intent to be bound by theory, it is believed that the mitigation of the spalling tendency occurs because the coeflicient of expansion of the ultimate protective layer is caused to be very low, more nearly matching that of the'base article.

There is given below a table of the coefiicients of thermal expansion of various iron-nickel alloys including some containing chromium.

Thermal expanszvity of iron-nickel alloys Means coefficients Means coefliclents Percent nickel or linear expan- Percent nickel of linear expansion 10 sionsXlO 0 8. 508-0. 00251! 5. 9. 901-0. 00067t 19. 9. 824+0. 002438 26. 13. 103+0. 02123! 10. 045+0. 0003 27. 11. 288+0. 02889t 11. 890+0. 00387t 28.7. 10. 387+0. 03004t 12. 661+0. 00550! 30.4. 4. 570+0. 01194t 12.2+1 Cr..." 11. 714+0. 00508! 31.4. 3. 395+0. 00885! 16.8+1 34.6.. 1. 373+0. 00237t 16.2 35.6. 0. 877+0. 0012' 21.3 37.3 3. 457-0. 0064713 34.8+l.5 Cr--- 3. 580-0. 00132! 39. 5. 357-0. 004486 35.7+1.7 Cr- 3. 373+0. 00165! 43.6 7. 992-0. 00273! 36.4+0.9 Cr--- 4. 433-0. 00392 A composition for the intermediate layer may be selected which will most closely match the coefficient of 7 thermal expansion of the base article to which the composition will be applied as a coating, bearing in mind that aluminum is to be alloyed and diffused into the coating. With base articles of the metals given above, especially good results are attained by using an intermediate alloy layer of approximately Invar compositions, i.e. about 35% nickel to iron. The percentage may be considerably varied, say from about 28% nickel to 50% nickel; but the series of intermediate coatings wit-hin the limitations set forth have been found to contribute unusual resistance to thermal fatigue to the final article and to mitigate spalling tendency.

The intermediate alloy layer may be deposited in vari- .ous ways; but it should be deposited as an alloy layer, and it should be deposited under conditions to insure high initial adherence to the base article. The intermediate alloy layer may be imposed upon the base article by known methods of vapor phase deposition which do not require description. Also the intermediate alloy layer may be deposited by electrodeposition. It may be stated, however,

that attempts to deposit the metals making up the intermediae alloy layer, along with the aluminum required to form the ultimate protective coating (as by vapor deposition) have not as yet proved successful.

Metal spraying may also be employed as a method of imposing the intermediate alloy coating on the base article, providing it is done under conditions productive of good initial adhesion.

The aluminum which is required for the formation of the ultimate protective coating may also be applied in various ways as by vapor deposition, metal spraying, or hot dipping in a molten aluminum bath. However, the aluminum should be applied as a separate and discrete layer and the ultimate diffusion caused to occur in a heat treatment as hereinafter set forth.

The following is a typical example of the use of the method on a gas turbine stator vane made of one of the metals or alloys above set forth for the base article:

The vane Was heated to burn off any oil or grease on its surfaces, and then was grit blasted all over to remove all oxide and other extraneous material. It was washed and placed in an electroplating bath consisting of an aqueous solution of ferrous chloride and nickelous chloride, these salts being present in the quantity of,200 grams each per liter.

At a pH of 2.7 to 3.0, and at temperatures in the range of 200 to 210 F., a current density of 100 amperes per square foot produced a codeposit of.nickel and iron in the approximate proportion of 35% nickel to 65% iron. The electroplating was continued until the deposit reached a thickness of .001 inch to .002 inch. Y

Following the electroplating the vane was heated to remove hydrogen, and the adhesion of the electrodeposit was checked before the aluminum*-was applied. The aluminum was applied in this instance by hot dipping the article bearing the nickel iron alloy coating into a bath of molten aluminum, maintaining it there'until its temperature had risen to the point where the molten aluminum adhered to its surface and then withdrawing it under known conditions to give a final cooled aluminum deposit of a thickness ranging from .00025 inch to .0025 inch. A final heat treatment within the range of approximately 1800 to 2000 F. was used to stabilize the product and form a diflfused coating for protection under service at high temperature.

Whereas it has hitherto been known that a thin aluminum coating presented a lesser spalling difiiculty, it is an advantage of the present invention that it permits the use of substantially thicker aluminum coatings, giving greater protection, with a decrease in spalling and a decrease in thermal fatigue. For example, the upper limit of thickness of an aluminum coating imposed directly upon the surfaces of some turbine blades made from an alloy of cobalt and manganese or tungsten was found to be about .00075 inch if non-commercial spalling was to be avoided. Vanes of the same base composition, however, when their surfaces were first coated with nickel-iron alloy of approximately Invar composition, were found capable of accepting aluminum coatings of a thickness of approximately .003 inch without spalling or other disadvantage upon the formation of the ultimate protective coating and use in high temperature service.

Modifications may be made in the invention without departing from the spirit of it. The invention having been described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

1. A process of making thermally resistant articles having a lessened tendency toward the spalling of a protective coating thereon which comprises making articles of metal chosen from a group consisting essentially of molybdenum, tungsten, niobium and alloys thereof, and alloys containing a preponderant amount of the foregoing and a metal selected from the group consisting or iron, nickel, and cobalt, coating said articles with a thin adherent layer of a ferrous alloy consisting essentially of from about 28% to about 50% of nickel, the remainder of said layer being iron, and thereafter adhering to the coated surfaces of said articles a layer of aluminum.

2. The process claimed in claim 1, including as a final step the heating of said articles to a temperature range of substantially l800 to 2000 F.

3. The process claimed in claim 1, wherein said ferrous alloy is imposed upon the surfaces of said articles by electroplating.

4. The process claimed in claim 1, wherein said ferrous alloy is imposed upon the surfaces of said articles by electroplating and wherein the aluminum is imposed upon the surfaces of said ferrous alloy by hot dipping.

5. The process claimed in claim 1, wherein the proportions of alloying ingredients in said therein layer of ferrous alloy are chosen to give a protective layer, when aluminum is diffused therewith, having a coefficient of thermal expansion close to that of the metal of said articles.

6. A metal article for high temperature service, comprising a base structure made of metal chosen from a group consisting of molybdenum, tungsten, niobium and alloys thereof, and alloys containing a preponderant amount of the foregoing and a metal chosen from a group consisting of iron, nickel and cobalt, said structure bearing on its surfaces a thin adherent layer of a ferrous alloy consisting essentially of about 28% to about 50% of nickel, the balance being substantially all iron, said article further having an adherent coating of aluminum covering the said thin layer of nickel-iron alloy.

7. The structure claimed in claim 6 bearing a diffused coating consisting of the named ingredients and formed by subjecting the coated article to a heat treatment at a temperature of substantially 1800 to 2000 F.

8. The structure claimed in claim 6 wherein the said ferrous alloy contains approximately 35 nickel to 65% iron and a minor quantity of chromium.

References Cited by the Examiner UNITED STATES PATENTS 2,484,118 10/49 Reynolds 29-196.2 2,490,543 12/49 Robertson 29-196.2 2,509,117 5/50 Wallace 29-196.2 X 2,682,101 6/54 Whitfield 29-198 X 2,752,265 6/56 Whitfield 29-l96.2 2,752,667 7/56 Schaefer 29-197 X 2,772,985 12/56 Wainer 29-196 3,000,755 9/61 Nanink 29-196.2 3,044,156 7/62 Whitfield 29-197 3,059,326 10/62 Jominy 29-197 DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner. 

6. A METAL ARTICLE FOR HIGH TEMPERATURE SERVICE, COMPRISING A BASE STRUCTURE MADE OF METAL CHOSEN FROM A GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN, NIOBIUM AND ALLOYS THEROF, AND ALLOYS CONTAINING A PREPONDERANT AMOUNT OF THE FOREGOING AND A METAL CHOSEN FROM A GROUP CONSISTING OF IRON, NICKEL AND COBALT, SAID STRUCTURE BEARING ON ITS SURFACES A THIN ADHERENT LAYER OF A FERROUS ALLOY CONSISTING ESSENTIALLY OF ABOUT 28% TO ABOUT 50% OF NICKEL, THE BALANCE BEING SUBSTANTIALLY ALL IRON, SAID ARTICLE FURTHER HAVING AN ADHERENT COATING OF ALUMINUM COVERING THE SAID THIN LAYER OF NICKEL-IRON ALLOY. 